Pet food prices spiked during the Great Recession and no one can figure out why

Last night I was searching the St. Louis Fed's economic data
website for cats, because what's better to do on a Friday night
at home than try to combine your two favorite things on the
internet, cats and charts?

The closest FRED gets to cat data is pet food inflation. I took a
peek and found something really interesting: pet food prices
spiked during the recession. It's not just a little spike. Prices
were up nearly 20% year-over-year in 2009.

Human food prices went up a lot during that time, too, but
nothing like the pet food prices.

I had a lot
of responses to the chart on Twitter (we all had very
exciting Friday nights), but no really good explanations for why
this might have happened.